Recent automobiles and vehicles have been built with on-board safety systems which include radar technologies for detecting a velocity of an object with respect to the vehicle so that a driver or collision-avoidance device can react accordingly. In one such technology, the Doppler effect is used to determine a relative velocity between the vehicle and the object. A transmitter at the vehicle transmits a modulated source signal, such as a chirp signal, towards the object at a selected frequency. The modulated source signal reflects off of the object and is received by a receiver at the vehicle. A frequency difference (known as the “Doppler frequency”) between the frequency of the source signal and the frequency of the reflected signal provides information about the relative velocity of the object with respect to the vehicle.
The frequency of the reflected signal is determined by sampling the reflected signal at a selected sampling rate and performing a Fast Fourier Transform (FFT) on the sampled signal to generate a frequency space that includes a representative peak. When the Doppler frequency is greater than a sampling frequency of the radar system, aliased frequencies appear in the frequency space, producing what is known as Doppler ambiguity. Doppler ambiguity can be resolved by lengthening the duration of the chirp signal in order to introduce additional (higher) Doppler sampling frequencies. However, this method requires additional control and processing and a corresponding increase in overhead and time expense. Accordingly, it is desirable to provide a method for resolving Doppler ambiguity using existing radar systems.